1. Field of the Invention
This invention relates to a receiving circuit for receiving signal inputted from the antenna with changing the path of the received signal in a receiver.
2. Description of the Prior Art
There is known a receiving circuit in which the signal path of the received signal can be changed in order to receive broadcasting waves in different frequency bands, such as the AM and FM bands, by a single antenna and switch the receiving circuits for those different frequency bands, or in order to operate a single receiving circuit with switching multiple antenna inputs, like the case of a diversity receiver.
Such a receiving circuit generally includes a pair of diodes connected in series. The connection point of those two diodes is connected to both an antenna output and an AM circuit, and the other terminal of one of the diodes is connected to an FM circuit. By appropriately changing the direction of the bias voltage applied to the serially-connected diodes, the signal received by the antenna is supplied either the AM circuit or the FM circuit. Thus, in a receiving circuit including a single antenna for a common use by an AM circuit and an FM circuit, the switching operation of the received signal can be readily controlled.
In such a receiving circuit, when the serially-connected diodes are applied with the reverse direction bias, a constant current flows the connection point of the two diodes if the characteristics of those diodes are ideal, i.e., ideally identical to each other. However, actually the characteristics of those diodes are not completely identical and hence the leak currents of different amount flow therethrough. This introduces the fluctuation of the reverse direction bias voltage. As a result, the AM circuit cannot receive constant voltage, and the distortion of the signal may take place in the AM circuit. In addition, the FM circuit becomes susceptible to the intermodulation interference due to the multiple FM transmission waves.
As described above, in a conventional receiving circuit, there are problems that the received signal is likely to be distorted due to the irregularity in characteristics of the diodes, such as the leak currents, and that it is difficult to maintain the desired signal receiving characteristics in both AM and FM circuits under the switching operation of them.